1. Field of the Invention
The present invention relates to a method of generating tool movement path data for machining three-dimensional surfaces, a device for generating the data, and machining method and system using the generated data, and more particularly to techniques for suitably determining a tool movement path depending upon the geometry of the three-dimensional surface to be generated by machining.
2. Discussion of the Related Art
It is widely practiced to machine three-dimensional surfaces such as those of a die, by using a machining tool for cutting or grinding, such as a ball end mill and other rotary tools. An example of a method for such machining is disclosed in JP-A-5-346814. This method consists of the steps of: (a) determining a tool constraining surface for constraining a machining tool to machine a three-dimensional surface, the tool constraining surface corresponding to the three-dimensional surface; (b) determining a tool path constraining plane for constraining a movement path of the above-indicated machining tool, such that the tool path constraining plane intersects the above-indicated tool constraining surface; (c) determining as the movement path of the machining tool a line of intersection between the above-indicated tool constraining surface and tool path constraining plane; and (d) moving the machining tool along the determined movement path, relative to the workpiece. FIG. 38 is a view for explaining such a machining method, wherein an NC data generating device 10 such as a CAM device obtains a line of intersection 16 between a tool constraining surface 12 and a tool path constraining plane 14, in the form of a three-dimensional curved line equation, which is supplied to an NC machine tool 18 as NC data (tool movement path data), so that a rotary machining tool 20 is moved along the three-dimensional curved line, for generating a desired three-dimensional surface 22. The tool constraining surface 12 is an offset surface which is offset from the desired three-dimensional surface 22 in a direction normal to the surface 22, by an amount equal to a radial dimension of the rotary machining tool (radius of curvature of the tip of the tool), and the movement path of the machining tool is a movement path taken by the center of the tool (the center of the sphere of the tool tip).
However, the conventional method described above does not necessarily permit adequate determination of the movement path of the machining tool for the three-dimensional surface to be generated by machining, leading to a possibility of drawbacks such as incapability to obtain high machining efficiency, abrupt change in the machining direction of the machining tool, and deterioration of the machining accuracy. Where a three-dimensional surface as shown in FIG. 9, for example, it is desired to smoothly move the tool along waves (follow the geometry) of the three-dimensional surface, as indicated at (a) by broken lines. In the conventional method, however, the tool movement path is constrained within the tool path plane, as indicated at (b) by broken lines, and the determined tool movement path may have abrupt changes in its direction. Where a conical shape as indicated in FIG. 10 is machined, it is desired to effect continuous machining from the small-diameter end to the large-diameter end, by moving the tool along a helical path, as indicated at (a) by broken line. In the conventional method, the machining is effected in steps along paths having respective different diameters, whereby the machining efficiency and accuracy (smoothness of the machine surface) are deteriorated.
Where a three-dimensional surf ace is machined while the attitude of the machining tool is controlled, the conventional method is adapted to determine the tool path defining points (cutter locations) at a predetermined spacing interval on the intersection line between the tool constraining surface and the tool path constraining plane, and obtain normal vectors of the tool constraining surface at the individual tool path defining points, so that the attitude of the tool is determined based on the obtained normal vectors. This method requires a lot of time to calculate the normal vectors, and suffers from limited freedom in determining the tool path defining points.
The present invention was made in view of the background art described above. It is an object of the present invention to suitably determine a movement path of the tool depending upon the geometry of a three-dimensional surface to be generated by machining. It is another object of the invention to generate tool movement path data including data of the tool attitude, in a short time and with increased freedom in determining the tool path defining points, even where the normal vectors are used to determine the tool attitude.